When I use your session config, all is working fine. This is on Ubuntu 16.04 with PHP7.
What is the value of your encryption key? Mine in example is like this:
PHP Code:
$config['encryption_key'] = hex2bin('5d3a06b1a1efeb861ad761fb8839794f');
I don't know that nginx would have anything to do with it, but my experience with it is limited. It's interesting that your session ID is returning NULL. I suppose you'd have to debug that and see what/why/where.